The invention relates to a heat transfer tube having an inner surface structure. The heat transfer tube is suited in particular for the evaporation of liquids from pure materials or mixtures on the inside of the tube. However, it also offers advantages for the condensation of vapors.
A world-wide competition in the field of heat exchangers, for example fin-tube heat exchangers (compare FIG. 1) for air-conditioning and refrigeration, demands high-performance heat transfer tubes, which are produced using little material (thus resulting in a low weight of the tube) and inexpensively in few tube forming steps. The heat transfer tubes are inserted into fin-tube heat exchangers, which can often be reversed between evaporation and condensation, and the tubes are thereby installed mostly horizontally into the fin-tube heat exchangers.
The state of the art includes a heat transfer tube according to:
(a) U.S. Pat. No. 5,332,034, in which during two successively occurring roll embossing steps fins of a uniform height are first roll embossed onto a strip, and during a second step notches are formed into the fins. The material displaced from the fins is thereby moved laterally of the fins into the troughs. The two-step fin forming process demands several embossing tools, which are arranged in series, and thus is less economical. In addition this two-step fin forming process does not achieve a reduction of the weight of the tube in spite of the forming of the notches. The notches in adjacent fins are aligned so that a second predestined flow direction in direction of the aligned notches results near the wall in addition to the troughs, which extend parallel to and between the fins. This second preferred direction serves indeed the transverse exchange between the troughs of the first-mentioned preferred direction, the additional creation of turbulence and the increase of the evaporation performance. However, on the other hand the existence of a second preferred direction makes the desired formation of a spiral flow in the area near the wall more difficult.
(b) DE-A-196 12 470, in which on the inner surface parallel and alternating (or also intersecting one another) high and low fins with notches additionally cut into the fins are formed. The notches of adjacent fins are here also aligned.
(c) DE-A-196 28 280, in which in peripheral direction of the tube the alignment of the fins is alternated in sections between two different directions. A spiral flow cannot form here due to the missing preferred direction and in contrast to the helix-shaped structures. This form of the structuring of the inner surface has proven to be little suited during evaporation since clearly lower evaporation performances are achieved than with tubes having a surface which provides a clear preferred direction for the flow near the wall. Whereas during condensation this type of surface structuring has proven to be advantageous.
(d) JP-A 4/158 193, in which in peripheral direction of the tube a differentiation is made in sections between areas of low and high fin heights. Of course, in addition to the first preferred direction in direction of the aligned fin elements a second one extending in longitudinal direction of the tube beyond the small fins is constructed, which very negatively influences in particular the evaporation performance since the flowing fluid is no longer necessarily forced into a spiral flow wetting also the upper half of the tube, but simply flows off in axial direction along the sections of lower fin height and above and beyond these small elements.
The purpose of the invention is to provide a heat transfer tube having an inner surface structure which combines the advantages of an evaporation performance, which is good or improved in comparison to the state of the art, and simultaneously has a reduced tube weight compared to the state of the art, and a reduced production expense effected by a reduction in the number of roll embossing steps.
The purpose is attained according to the invention in heat transfer tubes by the fins of each individual zone (Z1 to Zm) being arranged in longitudinal direction of the tube in any desired periodic combination and sequence of at least two fin heights (H1 to Hn, H1 greater than H2 greater than  . . .  greater than Hn) and extending at an angle of inclination with respect to the longitudinal axis of the tube, whereby adjacent zones (Z1 to Zm) border one another so that at the transition of two zones the fin sequence is staggered with respect to one another for at least one fin in longitudinal direction of the tube.
This results in the following advantages of the invention:
(1) Due to the alternating change between high and low fins in their longitudinal direction the possibility of a transverse exchange between the channels is offered over the fins of low height with a corresponding additional creation of a turbulence. However, the staggered arrangement of the fins of low height avoids a second and interfering preferred direction similar to the aligned arrangement of the notches disclosed in U.S. Pat. No. 5,332,034.
(2) A clear preferred direction of the flow near the wall exists precisely so that with the thus forced spiral flow a complete wetting of the entire tube circumference and especially of the upper half sections of the inner tube surface, is achieved. The wetting is needed for a good and improved evaporation performance. Whereas in the case of structures without a uniform preferred direction, as disclosed in DE-A-196 28 280, a drying of the upper sections of the tube circumference occurs and consequently a significant reduction of the evaporation performance.
(3) In contrast to the subsequent forming of the notches in a second embossing step, this structure can be created in one single embossing step so that, instead of the displacement of material out of the fins into the troughs, indeed a material savings and a weight reduction is achieved and in addition a reduction of the production expense through a reduction in the number of fin forming steps.
(4) Structures with an angle of inclination of the fins varying in zones offer mainly, with respect to the technique of shaping, important advantages since possibly occurring lateral forces, which are caused by the grooves and fins extending at an incline with respect to the direction of the strip, can be at least partially compensated for in the fin forming process, and the guiding of the strip is in this manner made easier. The heat transfer performance can be further increased by the edges, sharp-edge or also rounded projections and recesses, which edges are according to the invention provided additionally in the surface structure through the various heights, base widths, and cross-sectional shapes of the fins of varying height.
Through the various heights, base widths, and cross-sectional shapes of the fins of varying height additional edges, sharp-edge or also rounded projections and recesses are created in the surface structure and in the lateral flanks of the near wall troughs, which edges, projections and recesses serve to create a further turbulence and, in particular in the case of mixtures, to prevent the possible formation of temperature and concentration boundary layers and yet be available as additional nucleation sites. (Advantage over DE-A-196 12 470).
The manufacture of the heat transfer tube of the invention is based, for example, on the method described in greater detail hereinafter. Copper or a copper alloy are usually used as the material for the heat transfer tubes, however, the present invention is not limited in this manner. Rather any type of metal can be used, for example aluminum. A metallic flat strip is initially subjected to a one-step embossing step by being guided between an emboss roll with a surface design complementary to the structure of the invention and a support roll. One side of the flat strip receives thereby the structure of the invention, whereas the second side remains smooth or has also a structuring here not described in detail. Merely the strip edge areas of the first side, which edge areas are used for the subsequent welding, may possibly be differently structured or may even remain non-structured. The structured flat strip is after the embossing step formed into an open seam tube, is seam welded, and the tube, if necessary, receives in addition during a final drawing process the desired outside diameter. A possible influence on the heat-transfer ability of the heat transfer tube of the invention by the strip edge area, which surrounds the welding seam and which may be differently structured or remains even non-structured, is unimportant and can be neglected.
In the preferred embodiment of an emboss roll for the manufacture of the heat transfer tubes of the invention, the modular design of the emboss roll out of disks or rings is a further advantage of the invention. The design enables according to the modular concept a quick set-up and evaluation of many structure variations within the scope of a test scheme and a quick adaptation of the surface structuring to new fluids and changed operating conditions through a change of the number, form and (groove) geometry of the disks and rings or through the exchange of individual disks/rings.